We use this a lot in my house. There are 4 Airport Expresses in the house, all connected to speakers of some sort. Playing to all 4 of them at once works great on our network, so long as you are plugged in through ethernet, the wireless speeds just don't cut it, too much latency.

So we use NetTunes to remotely control the iTunes of a desktop mac that is always plugged in via ethernet. Much more convenient than repeatedly connecting your laptop to the network via ethernet, just for that one purpose.

NetTunes seems a little neglected these days, but it still works great. I think its basically a custom VNC server that is only serving the screen space containing the iTunes window. Clever.

p.s. it's also completely possible to use iTunes Music Sharing to stream music from the laptop to the desktop and then have the desktop ethernet connected mac stream the music to the 4 remote Expresses.